This invention relates generally to novel cyclobutabenzene derivatives, and more particularly, to acid or acid halide difunctionalized cyclobutabenzene derivatives which can be incorporated into any polymeric backbone structure as a substitute for, or in conjunction with, terephthalic acid or its acid chloride analog.
Recently, much effort has been expended toward producing high performance engineering thermoplastics with improved polymer microstructure in order to achieve specific desired polymer properties, such as strength, stiffness, long term dimensional stability, or acid/high temperature resistance. Some of this effort has been directed to incorporating cyclobutabenzene groups into known polymers.
The cyclobutabenzenes are useful because of a crosslinking reaction which can be thermally triggered resulting in ring-opening of cyclobutabenzene (BCB) to form the highly reactive o-quinodimethane intermediate at temperatures above 300.degree. C.: ##STR2## The o-quinodimethane intermediate is sufficiently reactive to homopolymerize through an addition reaction or by Diels-Alder dimerization. In either case, crosslinking should take place with little or no mass loss as the cyclobutane ring unfurls.
Previous use of the BCB functionality in polymer chemistry has been limited to structoterminal prepolymers (i.e., oligomers encapped with BCB). When the BCB functionality is used as an end-group in a polymer molecule, crosslinking occurs only at the end of the polymer molecules, and therefore, the degree of crosslinking is directly related to the molecular weight. When BCB functionalities are placed as a reactive pendant groups along the polymer backbone, the degree of crosslinking can be controlled. However, the resulting crosslinks are flexible and are not very ordered. This can lead to a lower modulus and disruption of crystallinity in the polymer, and hence, lower performance of polymer properties. Moreover, in both cases, the crosslinking function must be performed after polymerization is complete.
These approaches, therefore, suffer from limited processability and limited ability to vary crosslink density. There is, thus, a need for a structopendant crosslinking group (i.e., a crosslinking group placed directly in the backbone of the polymer as a monomeric unit), such that the reactive crosslinking functionality remains intact for secondary polymerization. This would permit greater control of the extent of crosslinking to achieve high performance polymers having particular desired properties. There is a further need for a structopendant crosslinking group which can be activated for crosslinking at any desired time during the processing, including subsequent to fiber formation, casting, or molding.
In all of the structoterminal prepolymers mentioned above, the aromatic BCB ring was simply monofunctionalized at the 4 position (the product accessible through electrophilic aromatic substitution). There is a need in the art for difunctionalized cyclobutabenzene derivatives which can be used as structopendant crosslinking groups. One obstacle encountered in incorporating the BCB functionality into the backbone of a polymer has been the difficulty in producing regioselective difunctionalization of the cyclobutabenzene unit. There is, therefore, a need for a method for regioselectively introducing a difunctionality onto the 3 and 6 positions of cyclobutabenzene.
Terephthalic acid (TA) is a difunctionalized monomer which is widely used in the production of high-performance aromatic polymers such as Poly{(benzo-[1,2-d;4,5-d']-bisoxazole-2,6-diyl)-1,4-phenylene}(PBZO), Poly{(benzo-[1,2-d;4,5-d']-bisthiazole-2,6-diyl)-1,4-phenylene}(PBZT), Poly(p-phenylene terephthalamide) (PPTA or Kevlar.RTM., a trademark of Dupont Chemical Company, Wilmington, Del.), and Poly(ethylene terephthalate) (PET). PPTA, PBZO, and PBZT, for example, are present state-of-the-art polymer materials for lightweight structural applications. Although these materials are strong and stiff, they are disadvantageously relatively weak in compression. The compression failure occurs by strain localization into well-defined kink bands. Detailed structural investigation of the kink bands using High Resolution Electron Microscopy revealed that both chain slip and chain bending or breaking are involved in this deformation mode.
PPTA, PBZO and PBZT are all bonded laterally in the solid state by weak secondary forces. There is a need in the art for a means of providing strong, covalent lateral crosslinks between the polymer molecules in these high-performance polymers to further improve their mechanical properties. There is, thus, a need in the art for an analog of TA which has an additional functionality having the ability to produce such strong, covalent crosslinks between polymer chains. There is a further need in the art for an analog of TA which can advantageously be substituted therefor to improve polymer properties without modifying existing processes.
It is, therefore, an object of this invention to provide difunctionalized cyclobutabenzene monomers and methods of making same.
It is another object of the invention to provide difunctionalized cyclobutabenzene monomers which can be substituted for existing corresponding monomers in existing polymers without substantially modifying the existing processes.
It is also an object of this invention to provide difunctionalized cyclobutabenzene monomers for controlled incorporation of a cyclobutane functionality into a polymer backbone to yield polymers of improved mechanical compressive strength and well as other improved properties.
It is still a further object of the invention to provide a method for regioselectively introducing a difunctionality onto the 3 and 6 positions of a cyclobutabenzene.